The present invention relates to a dampening water composition for lithographic printing and in particular to a dampening water composition excellent in anticorrosive ability against parts of a printing press made of copper and alloys thereof or cast iron as well as plated parts thereof and a lithographic printing method using the same.
The lithographic printing is a printing system which makes the most use of the properties that water and an oil are not intermingled with one another. Thus, the printing plate used in such system comprises a surface region which receives water, but repels an oil-based ink and a surface region which repels water, but receives an oil-based ink, wherein the former serves as a non-image area, while the latter serves as an image area. A desensitizing gum serves to increase the surface-chemical difference between the image and non-image areas and hence the ink repellency of the non-image area and ink-receptivity of the image area by wetting the non-image area with a dampening water containing the desensitizing gum.
In the practice of printing, a dampening water is first supplied to the plate surface through rollers for applying water and then a lithographic ink is applied onto the plate surface through three or four rollers for applying ink. Therefore, the dampening water exists on mixing rolls of the printing press in the form of drops or a layer of water since the dampening water is transferred thereto through the lithographic ink on the ink-applying rollers which come in contact with the plate surface.
For this reason, it is sometimes observed that a sufficient amount of the water-repellent lithographic ink is not adhered to the mixing rolls of the printing press. This phenomenon is generally called "roller stripping". To suppress the occurrence of this phenomenon as much as possible, there has widely been used a printing press in which a part of the mixing roll is made of copper or a copper alloy having high affinity for oils. When such a printing press is employed in printing, an ink is distributed only to the image areas on a lithographic printing plate, while the hydrophilic state of the non-image area is maintained and the ink is transferred to paper through a rubber blanket. An impression cylinder (which is made of cast iron and nickel- or chromium-plated or burnished) comes in contact with the blanket during this operation.
Thus, when the printing is performed using a printing press equipped with copper or copper alloy-plated rolls and/or an impression cylinder, the copper or copper alloy-plated rolls and/or the impression cylinder often suffer corrosion or rust depending on the kinds or amounts of additives incorporated into the dampening water. Therefore, there has long been a desire for the development of a dampening water which can prevent the corrosion of these parts of the printing press.
Under such circumstances, there have been proposed various methods for solving these problems. For instance, U.S. Pat. No. 4,548,645 discloses the use of a polycarboxylic acid and salts thereof with organic bases such as amine salts. In addition, DEOS No. 3,536,485 discloses the use of a dampening water to which a thickener, a citrate buffer, a surfactant and copper ions are added to eliminate the problem of corrosion. European patent application Ser. No. 0108883 discloses a method for preventing the corrosion which uses a dampening water composition containing 1H-benzotriazole. These dampening water compositions are effective for preventing the corrosion of the parts made of copper or copper alloys, but are not always effective for preventing the corrosion of cast iron parts and nickel-plated cast iron parts.